1. Field of the Invention
The present invention relates to an Al.sub.2 O.sub.3 (alumina) based ceramic material and a method of manufacturing the same. In particular, the present invention concerns an Al.sub.2 O.sub.3 based ceramic material suitable for tools requiring high strength, toughness and wear resistance, such as cutting tools and a die extrusion plug, and mechanical parts requiring wear resistance, toughness and shock resistance, such as a pump valve and a mechanical seal; and a method of manufacturing the same.
2. Description of the Related Art
An Al.sub.2 O.sub.3 based ceramic material, having a high strength and being excellent in corrosion resistance, oxidization resistance and wear resistance, has been known to be suitable for mechanical parts and jigs/tools. However, it is inferior in strength and toughness to a Si.sub.3 N.sub.4 based ceramic material which has been extensively used as an engineering ceramic material.
Various attempts have been made to improve the strength and toughness of an Al.sub.2 O.sub.3 based ceramic material. One is to add SiC whiskers to an Al.sub.2 O.sub.3 based ceramic material, as disclosed in U.S. Pat. No. 4,543,345. This method is successful to obtain the excellent toughness; however, it presents a problem in the manufacturing process. Namely, in this method, an Al.sub.2 O.sub.3 based ceramic material added with SiC whiskers (hereinafter, referred to as "SiC whiskers containing Al.sub.2 O.sub.3 based ceramic material") is manufactured by hot-pressing, and is difficult to be formed in a complex shape or in a large size.
To solve this problem, a pressureless-sintering process has been attempted for a SiC whiskers containing Al.sub.2 O.sub.3 based ceramic material. For example, Terry N. Tigers and others disclose a method in [J. Am. Ceram. Soc., 73[5] (1990) 1440-1442], wherein the pressureless-sintering is performed while the aspect of SiC whiskers and the kinds of sintering auxiliary agents are changed. Young-Wook Kim and others propose a method in [J. Mat. Sci., 26 (1991) 1316-1320], wherein the pressureless-sintering is performed using a special process of precipitating aluminum hydroxide in a green compact before sintering. On the other hand, the present inventors disclose a method in Unexamined Japanese Patent Publication No. HEI 4-124058, wherein Al.sub.2 O.sub.3 is added with SiC whiskers and a sintering auxiliary agent composed of oxides of Mg, Si, Ca, Ti, Zr, Cr, Ni, Y and rare earth elements. In addition, either of these pressureless-sintering methods is performed at a temperature between 1500.degree. and 1900.degree. C. in an inert gas atmosphere such as argon for preventing the reaction between SiC whiskers and the atmospheric gas.
The above-described prior art methods using pressureless-sintering, however, has a disadvantage in that a sufficiently densified sintered body is very difficult to be obtained because the sintering is obstructed by SiC whiskers.
To cope with the disadvantage, there may be considered methods of: for example, reducing the added amount of SiC whiskers to the extent that a sufficiently densified sintered body can be obtained without any obstruction of the sintering caused by SiC whiskers; and of extremely reducing the aspect ratio of SiC whiskers or increasing the added amount of a sintering auxiliary agent as described in [J. Am. Ceram. Soc. 73[5] (1990) 1440-1442]. However, these methods tend to significantly lower the strength and toughness of an Al.sub.2 O.sub.3 sintered body. Accordingly, a technique has been required, which is capable of manufacturing, a SiC whiskers containing Al.sub.2 O.sub.3 based ceramic material composed of a sintered body being sufficiently densified and having a sufficient strength and toughness, by pressureless-sintering.
In addition, the manufacturing method by hot pressing described in U.S. Pat. No. 4,543,345 can manufacture only a product having a simple shape and a small size, and further it takes a lot of labor.